Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. The various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF). Commercially employed fluoropolymers also include fluoroelastomers and thermoplastic fluoropolymers.
Several methods are known to produce fluoropolymers. Such methods include suspension polymerization as disclosed in e.g. U.S. Pat. No. 3,855,191, U.S. Pat. No. 4,439,385 and EP 649863; aqueous: emulsion polymerization as disclosed in e. g. U.S. Pat. No. 3,635,926 and U.S. Pat. No. 4,262,101; solution polymerization as disclosed in U.S. Pat. No. 3,642,742, U.S. Pat. No. 4,588,796 and U.S. Pat. No. 5,663,255; polymerization using supercritical CO2 as disclosed in JP 46011031 and EP 964009 and polymerization in the gas phase as disclosed in U.S. Pat. No. 4,861,845.
Currently, the most commonly employed polymerization methods include suspension polymerization and especially aqueous emulsion polymerization. The aqueous emulsion polymerization normally involves the polymerization in the presence of a fluorinated surfactant, which is generally used for the stabilization of the polymer particles formed. The suspension polymerization generally does not involve the use of surfactant but results in substantially larger polymer particles than in case of the aqueous emulsion polymerization. Thus, the polymer particles in case of suspension polymerization will quickly settle out whereas in case of dispersions obtained in emulsion polymerization generally good stability over a long period of time is obtained.
An aqueous emulsion polymerization wherein no surfactant is used has been described in U.S. Pat. No. 5,453,477, WO 96/24622 and WO 97/17381 to generally produce homo- and copolymers of chlorotrifluoroethylene (CTFE). For example, WO 97/17381 discloses an aqueous emulsion polymerization in the absence of a surfactant wherein a radical initiator system of a reducing agent and oxidizing agent is used to initiate the polymerization and whereby the initiator system is added in one or more further charges during the polymerization. So-called emulsifier free polymerization has further been disclosed in WO 02/88206 and WO 02/88203. In the latter PCT application, the use of dimethyl ether or methyl tertiary butyl ether is taught to minimize formation of low molecular weight fractions that may be extractable from the fluoropolymer. WO 02/88207 teaches an emulsifier free polymerization using certain chain transfer agents to minimize formation of water soluble fluorinated compounds. An emulsifier free polymerization is further disclosed in RU 2158274 for making an elastomeric copolymer of hexafluoropropylene and vinylidene fluoride.
Notwithstanding the fact that emulsifier free polymerizations are known, the aqueous emulsion polymerization process in the presence of fluorinated surfactants is still a desirable process to produce fluoropolymers because it can yield stable fluoropolymer particle dispersions in high yield and in a more environmental friendly way than for example polymerizations conducted in an organic solvent. Frequently, the emulsion polymerization process is carried out using a perfluoroalkanoic acid or salt thereof as a surfactant. These surfactants are typically used as they provide a wide variety of desirable properties such as high speed of polymerization, good copolymerization properties of fluorinated olefins with comonomers, small particle sizes of the resulting dispersion can be achieved, good polymerization yields i.e. a high amount of solids can be produced, good dispersion stability, etc. However, environmental concerns have been raised against these surfactants and moreover these surfactants are generally expensive. In particular, perfluoro-alkanoic acids having 8 or more carbons are known to be bioaccumulating. Alternative surfactants to the perfluoroalkanoic acids or salts thereof have also been proposed in the art for conducting the emulsion polymerization of fluorinated monomers.
For example, surfactants of the general formula Rf—C2H4—SO3M, wherein Rf represents a perfluorinated aliphatic group and wherein M represents a cation, have been disclosed in U.S. Pat. No. 5,789,508, U.S. Pat. No. 4,025,709, U.S. Pat. No. 5,688,884 and U.S. Pat. No. 4,380,618.
U.S. Pat. No. 5,763,552 discloses partially fluorinated surfactants of the general formula Rf—(CH2)m—R′f—COOM wherein Rf represents a perfluoroalkyl group or a perfluoroalkoxy group of 3 to 8 carbon atoms, R′f represents a perfluoroalkylene of 1 to 4 carbon atoms and m is 1-3.
U.S. Pat. No. 4,621,116 discloses perfluoroalkoxy benzene sulphonic acids and salts thereof in the aqueous emulsion polymerization of fluorinated monomers.
U.S. Pat. No. 3,271,341 teaches perfluoropolyethers of the general formula:F—(CF2)m—O—[CFX—CF2—O]n—CFX—COOAwherein m is 1 to 5, X is F or CF3, A is a monovalent cation and n is 0 to 10. The perfluoropolyethers are taught as emulsifiers in the emulsion polymerization of ethylenically unsaturated monomers.
US Publication No. 2005/0090613 discloses fluorinated polyethers of the formula:F—(CF2)m—O—[CFX—CF2—O]n—CFX—COOAwherein m is 3 to 10, X is F or a perfluoroalkyl group, n is 0, 1 or 2 and A is the counter ion of the carboxylic anion. These polyethers are taught as emulsifiers in the emulsion polymerization of fluorinated olefins.
The use of perfluoropolyethers having neutral end groups in an aqueous emulsion polymerization is disclosed in U.S. Pat. No. 4,864,006, U.S. Pat. No. 4,789,717 and EP 625526. For example U.S. Pat. No. 4,864,006 and EP 625526 disclose the use of microemulsion prepared from perfluoropolyethers having neutral end groups in an aqueous emulsion polymerization of fluorinated monomers. In a particular embodiment, a certain perfluoropolyether having carboxylic end groups is taught to emulsify the neutral perfluoropolyether.
EP 1,334,996 discloses certain perfluoropolyethers having carboxylic acid groups or salts thereof at both end groups, i.e. the perfluoropolyethers are bifunctional. The perfluoropolyethers are taught for use in aqueous dispersions of fluoropolymers and in the preparation of such dispersion by aqueous emulsion polymerization.
WO 00/71590 teaches the use of a combination of perfluoropolyether surfactants having a carboxylic acid group or salt thereof with a fluoroalkyl carboxylic acid or sulphonic acid or salt thereof. It is taught that the perfluoropolyether surfactants on their own are not very powerful surfactants.
Still further, WO 05/03075 discloses certain fluorinated polyether surfactants for emulsion polymerization of fluorinated monomers.